It is well known that certain characteristics of bitumens can be improved by modification by or addition of polymeric materials. For example, European Application No. 88202562 (Publication No. 317,025, published May 24, 1989, assigned to Shell Internationale Research Maatschappij BV) discloses a bitumen composition useful in road paving applications containing an assymetric radial block copolymer which exhibits increased toughness and tenacity. Recently published PCT Application No. FR89/00453 (Publication No. WO 90/02776, assigned to Societe Nationale Elf Aquitaine) discloses bitumens modified with a copolymer of styrene and a conjugated diene and a coupling agent such as sulfur.
A problem often encountered with bitumen-polymer mixtures is an incompatibility of the bitumen and polymer components. Bitumens and most polymers, especially polyolefins such as polyethylene, are not readily miscible with each other in a molten state.
One approach to overcoming this problem has been the use of another additive to form a stabilized bitumen gel as described in, for example, U.S. Pat. No. 4,018,730 (issued Apr. 19, 1977 to McDonald). This approach has the disadvantage of providing a thickened or jellied material, the higher viscosity of which has less desirable working characteristics than is desirable for conventional uses of bitumens.
More closely related to bituminous compositions obtainable with the present invention are those disclosed in, for example, U.S. Pat. No. 4,314,921 (issued Feb. 9, 1982 and assigned to Novophalt SA) and German Offenlegungsschrift 39 20 878 (published Jan. 4, 1990 and assigned to Novophalt SA). These documents disclose methods for the physical mixture i.e., homogenization of molten bitumens and thermoplastic polymers such as polyethylene. Such specialized mixing methods have heretofore been found to be necessary because of the difficulty of obtaining adequate dispersion of polymer components in the bituminous phase to provide the desired qualities in the working material produced. As described in U.S. Pat. No. 4,314,921, shearing forces which degrade the polymer are apparently necessary. Further, there is a tendency of the homogenized bitumens and polymers to undergo gross phase separation even after such homogenization. Commercial applications of the Novophalt process thus include the addition of paving components such as sand and gravel to the homogenized mixture within a relatively short period of time after the homogenization process is complete.
U.S. Pat. No. 4,154,710 of Maldonado et al. (issued May 15, 1979 to Elf Union) discloses a bitumen modified by heating bitumen in the presence of polyisobutene or a mixture of fatty acid esters (esters of oleic acid, palmitic acid, stearic acid with higher alcohols such as lanosterol, cholesterol, isocholesterol) consisting of a natural extract of mutton suet in the presence of sulphur. Apparently stable mixtures were obtained with the admixture of polymers such as, for example, isobutene-butadiene copolymer, ethylenecylcopentadiene copolymer, polybutene-polyisobutene polymers.